1. Field of the Invention
The invention relates to a pneumatic hand tool, more particularly to a pneumatic hammer drill.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional pneumatic hammer drill includes a handle body 10, a pneumatic cylinder 12, an air valve 13, and a tool 14. The handle body 10 defines a cavity 111 at an upper portion thereof, and has a lower handgrip portion 11. The pneumatic cylinder 12 is fitted into the cavity 111, and includes an externally threaded front end 1213 extending outwardly from the cavity 111, a spring element 122 engaged threadedly to the front end 1213, a pressure chamber 1211, a piston 123 inserted slidably into the pressure chamber 1211, and an air passage 1212 in fluid communication with the pressure chamber 1211 and the air valve 13. Highly compressed air is introduced into the air passage 1212 via the lower handgrip portion 11. The spring element 122 has a limiting end 1221 extending outwardly and curvedly from a front end thereof.
The air valve 13 is disposed in the cavity 111 adjacent to the pneumatic cylinder 12, and includes a front valve casing 131, a rear valve casing 132, a diaphragm 133, and a plurality of positioning pins 134. The front valve casing 131 is provided with a valve orifice 1311 to permit air flow therethrough, and a plurality of positioning holes 1312 around the valve orifice 1311. The rear valve casing 132 is provided with a valve orifice 1323, a valve seat 1321 disposed around the valve orifice 1323 and receiving the diaphragm 133 therein, and a plurality of fixing holes 1322 around the valve seat 1321. The positioning pins 134 interconnect the front and rear valve casings 131, 132, and have front ends inserted respectively into the positioning holes 1312 in the front valve casing 131, and rear ends inserted correspondingly into the fixing holes 1322 in the rear valve casing 132, thereby aligning the holes 1312, 1322 in the front and rear valve casings 131, 132.
The tool 14 has an annular protrusion 142 proximate to a rear portion 141 thereof and located between a front end face of the pneumatic cylinder 12 and the limiting end 1221 of the spring element 122.
The compressed air flows through the valve orifices 1311, 1323 from the air passage 1212 in the pneumatic cylinder 12, and enters a rear side of the pressure chamber 1211 so as to push forwardly the piston 123, which in turn, strikes the tool 14 so that the tool 14 produces a hammering force on a workpiece. The tool 14 cannot be released from the pneumatic cylinder 12 as the annular protrusion 142 thereof is engaged with the limiting end 1221 of the spring element 122. When the compressed air enters a front side of the pressure chamber 1211 via the air passage 1212, the piston 123 is pushed rearwardly toward the front valve casing 131 so that the tool 14 is restored to its original position through a restoring force of the spring element 122. Hence, by moving the piston 123 reciprocatingly in the pressure chamber 1211 through such supply of the compressed air, a hammering action of the tool 14 is effected.
Although the aforementioned conventional pneumatic hammer drill can achieve its intended purpose, it has many drawbacks. Firstly, since a front end face of the front valve casing 131 and a rear end face of the piston 123 are generally flat, and since the impact of the piston 123 against the front valve casing 131 is strong when the compressed air pushes the piston 123 rearwardly toward the front valve casing 131, internal components of the hammer drill are prone to damage over time. Further, because of the strong impact force, an operator's hand is easily fatigued and may even become injured. Moreover, the strong impact produces a loud noise which is unpleasant and may even adversely affect the sense of hearing of the operator. Additionally, the positioning holes 1312 in the front valve casing 131 must be aligned with the fixing holes 1322 in the rear valve casing 132 and at the same time with the air passage 1212 in the pneumatic cylinder 12 so as to permit air flow into the air valve 13. However, since such alignment is not easily achieved, overall assembly of the conventional hammer drill is made complicated.